A diaphragm wall is made by casting a series of concrete panels, which may be reinforced, in excavated trenches as described, for example, in EP 0 101 350 and EP 0 402 247. In some cases, alternate `primary` panels are constructed first, followed by infill (i.e. closing) `secondary` panels. The installation sequence would be, for example, panels 1, 3, 5, 7, 9, 11 etc. followed by panels 2, 4, 6, 8, 10 etc. In other cases, only a few `primary` panels are first constructed, for example panels 1, 10 and 20. Following this, a series of `continuity panels 2, 11, 3, 12 etc. are installed, with the diaphragm wall being completed by `closing` panels 9 and 19. All primary panels require the use of shutters at each edge of their respective trenches in order to provide well-defined edges to each panel so as to ensure that the joints between adjacent panels may be made watertight. Continuity panels, in contrast, require only one shutter at the edge of the trench furthest away from the previously cast panel. No shutters are required for closing panels. The shutters are conventionally known as `stop-ends`, and provide the concrete at each vertical edge of the panels with a predetermined shape.
In order to reduce water leakage across the joints between panels, it is possible (with some types of stop-end) to install a waterbar between adjacent panels. A waterbar comprises a strip of suitable material, for example rubber, PVC or steel, which has one longitudinal edge embedded in the edge of one cast panel and the other longitudinal edge embedded in the adjacent panel. Preferably, the waterbar extends over substantially the entire height of the diaphragm wall. Such a waterbar may be installed by employing a stop-end provided with a slot in its face into which the waterbar may be fitted, with about one half of its width remaining exposed. When concrete is poured into the trench on this side of the stop-end and allowed to set, the stop-end may subsequently be removed so as to leave approximately half the waterbar embedded in the resulting concrete panel. When the next panel is cast, the remaining exposed portion of the waterbar will become embedded in concrete, thereby resulting in a seal between the two adjacent panels. Typical waterbars have beaded longitudinal edges, giving the waterbar a dumb-bell shaped cross-section, with an optional central bulb.
This type of waterbar, however, does not necessarily guarantee watertightness at panel joints. This is because the known varieties of waterbar rely on the fact that concrete shrinks slightly upon setting. An element which is completely surrounded by concrete will be gripped all round as the concrete shrinks, but the beaded edges of the known varieties of waterbar are not completely surrounded by concrete due to the presence of the central part of the waterbar. Accordingly, there is a potential risk of water leakage. Furthermore, installation conditions are in practice far from perfect, partly because the waterbar is installed "blind" under a bentonite mud containing suspended sand and the like, and partly because the concrete is cast without vibration, which means that the bentonite and/or sand and the like may not be fully removed from the edges of the waterbar.
Waterbars made of hydrophilic materials have been used at construction joints in conventional "above ground" concrete structures. The hydrophilic material is placed at the joint in dry conditions. If and when water enters the joint, the hydrophilic material will swell, thereby forming a seal between the two adjacent concrete members
By contrast, the use of hydrophilic waterbars in diaphragm wall construction presents a number of problems, not least because installation takes place in an aqueous environment, and the part of the waterbar to be incorporated into the second panel will swell before the trench for that panel is excavated and the concrete cast. One way of approaching this problem is to use hydrophilic waterbars provided with a protective, e.g. sugar, coating, which can theoretically give a delay of several days before swelling occurs. This, however, is often unreliable, mainly because the coating resembles a sugar glaze which is cracked when the waterbar is flexed, is easily damages in handling, and has inherent imperfections in the coating, all of which will lead to premature swelling of portions of the waterbar. Moreover, in many installations it is not unusual to leave more time between the casting of adjacent panels than can be accommodated by the protective coating; indeed, intervals of up to thirty days are not uncommon.